Embodiments of the present invention relate to processes for preparing active catalysts, including supported catalysts, suitable for the polymerization or copolymerization of olefins and the products, including polymer polymerization products produced in accordance with the processes described. Such products are typically industrially and commercially useful polymers.
Catalyst compositions comprising organometallic complex compounds generally including metallocenes, in combination with an activator, such as aluminoxane, are known for the polymerizing olefins and such catalysts are generally considered valuable in view of their good activity, in other words, the ability to produce a high quantity of olefin polymer for each gram of catalyst. Properties of the polymers produced using such catalysts can be affected not only by polymerization process conditions but also by characteristics of the catalyst composition such as its chemical composition, morphology and the like. In particular, it is recognized that further improvements are needed to identify catalysts and catalyst systems that are active and capable of producing useful polymers such as high molecular weight polymers possessing or exhibiting a low melt index.
Use of catalyst systems of the present invention in various polymerization processes can provide products which exhibit different properties. In the case of olefin polymers, their suitability for particular applications depends, for example, on the nature of the monomers on which they are based and on the choice and ratio of comonomers and the typical physical parameters which characterize the polymer, such as number average (Mn) or weight average (Mw) molecular weight, molecular weight distribution (expressed, for example, as Mw/Mn), degree of branching, degree of crosslinking, crystallinity, density, presence of functional groups in the polymer and the like, as well as on polymer properties resulting from the process used, such as the content of low molecular weight impurities the presence of catalyst residues, etc., as well as costs.
Various classes or types of single site or metallocene catalyst systems produce polymers having generally recognized properties derived from the uniformity of a catalytically active site. Thus, such catalysts typically produce polymers having narrow molecular weight distributions (Mw/Mn equal to about 2 or less) and uniform polymer architecture. The uniformity of single-site catalysts typically results in uniform polymer chains in terms of, for example, the distribution of short chain branching throughout the polymer chains. Additionally, the response of single-site catalysts to polymerization conditions or the use of comonomers or chain terminating agents, such as hydrogen, allows for tailoring the polymer architecture, and thus properties of the polymers produced.
In contrast, traditional Ziegler-Natta (Z-N) catalysts, based on titanium (Ti) and magnesium (Mg), typically produce broader molecular weight distribution polymers, for example Mw/Mn=4 to 6. The active catalytic sites of such Z-N catalysts are not uniform and the different sites present typically react differently to polymerization conditions, as well as to comonomer or the use of hydrogen. Thus, the resulting polymer chains do not exhibit a uniform architecture. Thus, controlling polymer properties can be difficult because of the differing response of the mixture of active sites in Z-N catalysts to varying conditions and monomers or reactants. For example, film produced using linear low density polyethylene (LLDPE) produced with traditional Z-N catalysts may be inferior to film based on LLDPE produced with metallocene catalysts (also referred to as m-LLDPE) with respect to, for example, the efficiency of comonomer usage to achieve a desired density, or film properties such as film clarity or puncture resistance. However, one advantage of traditional Z-N catalysts over metallocene catalysts is their ability, under typical polymerization conditions, to produce polymers having sufficiently high Mw to be useful in film processing, where melt strength is critical for processing, or having other desirable polymer properties or both. In the absence of added hydrogen during polymerization, Z-N catalysts are capable of producing ultra-high molecular weight polyolefin polymers or resins, having values of Mw≧1,000,000. For resins that typically require lower Mw, such as those used to produce LLDPE film, hydrogen can be introduced to the polymerization reactor where it functions as a chain terminating agent, thereby lowering Mw. In contrast, many metallocene catalysts, especially very active metallocenes, can not produce polymers having the desired molecular weight for the target application, e.g., LLDPE film, under practical polymerization conditions. Thus there remains a need for improved catalyst systems capable of producing polymers having a uniform polymer architecture, particularly at desirably high molecular weight as measured, for example, by a low melt index. The present invention addresses such needs as well as others.